The Early Expansion of Cluster Cores

TL;DR

This study reveals that young star clusters rapidly expand their cores within the first 20 million years, affecting their density and dynamical evolution, based on observations and literature data.

Contribution

It provides new observational evidence for rapid core expansion in young clusters and discusses its implications for cluster dynamics and evolution.

Findings

Clusters expand rapidly during first 20 Myr

Core sizes increase with age across multiple datasets

Past densities and relaxation times were significantly higher

Abstract

The observed properties of young star clusters, such as the core radius and luminosity profile, change rapidly during the early evolution of the clusters. Here we present observations of 6 young clusters in M51 where we derive their sizes using HST imaging and ages using deep Gemini-North spectroscopy. We find evidence for a rapid expansion of the cluster cores during the first 20 Myr of their evolution. We confirm this trend by including data from the literature of both Galactic and extra-galactic embedded and young clusters, and possible mechanisms (rapid gas removal, stellar evolutionary mass-loss, and internal dynamical heating) are discussed. We explore the implications of this result, focussing on the fact that clusters were more concentrated in the past, implying that their stellar densities were much higher and relaxation times correspondingly shorter. Thus, when estimating if a particular cluster is dynamically relaxed, (i.e. when determining if a cluster's mass segregation is due to primordial or dynamical processes), the current relaxation time is only an upper-limit, with the relaxation time likely being significantly shorter in the past.